Hierarchial wiring structures are now commonly provided in modern local area network installations. Such structures include various different levels to provide a fanning out of the network from a centralized core. Client-server computing technologies drive this type of structure. High speed servers are commonly located at a central switch. The client end-user hosts are located in a manner similar to "leaves" of the tree. Fully functional switches (or bridges) are for the most part deployed at the various levels to provide the fanning out of the network from the centralized core.
A traditional media access control (MAC)--layer bridge treats all ports as equal peers. Such a bridge is based on for example two shared segments wherein the bridge considers if packets should be transferred from one segment to the other segment. The bridge assumes that each port is connected to one of the shared LAN segments. The segments consist of more than one end station or host. The bridge performs local-packet filtering (also known as bridge filtering). The bridge forwards packets to the appropriate destination port based on the address table entries. Every packet entering a bridge must have a source plus destination addresses. The source and destination addresses are interrogated based on a table. The use of such a table for looking up destination addresses and/or source address is very central processing unit (CPU) intensive or requires complex application specific integrated circuits (ASICs).
Switches are known which provide the connection of additional or multiple ports to a bridge (greater than the two segments as discussed above). Instead of considering this a bridge connecting two segments, such a multiple port device may also be referred to as a switch. With such a bridge/switch it is possible to connect a single host to each bridge port. In such an arrangement, the segments are not all shared as various hosts are connected to various different ports of the switch.
With such a switch, all ports remain equal and traffic is more or less still distributed. With the inclusion of a server connected to a port of the switch, some level of uni-directional traffic flow (data packet flow) is possible. All bridge functions remain intact with this type of bridge/switch and these functions are required for this flat wiring scheme.
The basic bridge connecting two segments and the switch connecting a segment and multiple ports (including single host ports and server ports) is the basis for known hierarchial wiring structures. With such a hierarchial implementation, various switching levels provide a fanning out of the network from a centralized core. The centralized core may be considered level 0 with a switch which operates as the core. Next, several level 1 switches may be connected downstream from the core. Each level 1 switch operates as a feeder to the level 0 switch and interconnects multiple level two switches which are below the level 1 switch. The level two switches are the direct connection to the host, namely these provide the end user connectivity to the network. Level 2 switches exist at the edge (the leaves) of the network and have the greatest numbers due to the fan-out nature of the structure. With known hierarchial levels, the layer 0 switch somewhat closely approximates the traditional bridge model, with peer-peer ports and distributed data flows being the norm. That is, various hosts share a segment connected to a port of the layer 0 switch. The CPU or ASIC provides the interrogation of the source and destination addresses via a table, to determine what segments to switch forwards packets to.
The layer 1 switch and the hierarchial structure is more of a feeder than a traditional bridge, although it maintains equivalent link speeds and is not optimized for unidirectional traffic. Further, each port of a level 1 switch is attached to a logically shared segment consisting of all downstream end stations, exactly as in the layer 0 switch.
The use of the layer 2 switch is the most divergent from the traditional bridging model. It is used acting virtually 100% as a traffic feeder with predictable upstream/downstream traffic flows. Each port is connected to only one end station so that each attached segment is not shared. The port speeds are asymmetric. End-station segments are at a speed of 10/100 Mbps and the uplink is at 1 Gbps. Although peer to peer traffic through the level 2 switch is possible, such peer to peer traffic is expected to be nominal or non-existent.
With such a level 2 switch in a hierarchial structure, the traditional any port--any port bridging model, with interrogation of destination address and source address via a table, is under-utilized. Most all of the ingress traffic flows upward toward the server. Virtually all ingress traffic flows down and out to the ports.